MIMO (multiple-input multiple-output) technology means a method of improving data transfer efficiency by adopting multiple transmitting antennas and multiple receiving antennas instead of using a single transmitting antenna and a single receiving antenna. In particular, the MIMO technology is the technology for improving capacity or performance of a transmitting or receiving end of a wireless communication system by using multiple antennas. In addition, the MIMO technology can be referred to as a multi-antenna technology.
To support MIMO transmission, a precoding matrix can be applied to properly distribute transmission information to each antenna according to a channel status and the like. In the conventional 3GPP (3rd generation partnership project) LTE (long term evolution) system, maximum 4 transmit antennas (4Tx) are supported for downlink transmission and a related precoding codebook is defined.